Fluorinated chromia reactions

Fluorinated chromia used to catalyze the isomerization reaction of CHF2CHF2 to CF3CH2F, also exhibited evidence for the importance of chromium in higher oxidation states. FTIR spectroscopic measurements of CO adsorption confirmed the occurrence of Cr4+ and Cr5+ on the surface of chromia catalysts before being used [52]. During the activation, Cr4+ and Cr5+ sites were reduced and enhanced activity of the catalyst was observed. The reaction pathway proposed for isomerization involves the formation of hydrogen fluoride due to the degradation reaction of the fluoroalkane. 

In neither case was it possible to propose definitive mechanisms due to the complexity of the systems in the 7-alumina study, it is suggested that adsorption-desorption processes are slow relative to rapid dismutation between two adsorbed species [105], while from the chromia study mono-molecular halogen exchange reactions with metal halide surface sites are indicated [38], The latter mechanism is reminiscent of the halogen exchange model proposed [95] for C2 CFCs on fluorinated chromia. 

The following discussion examines the role of the two materials under consideration. Section 2 reviews fundamental issues connected with the role of fluorinated chromia as an active catalyst for a variety of reactions Section 3 considers physicochemical attributes of the acid site distribution associated with rj-alumina catalysts as applied to a single reaction, namely the synthesis of methyl chloride via the hydrochlorination of methanol. 

Figure 7.3 Two Possible Pathways for Catalytic Reactions at a Fluorinated Chromia Surface Reversible Halogen Exchange, that is, F-for-CI and CI-for-F Oehydrochlorination Followed by...

Equations (1 )-(3) represent a synthesis scheme from the patent literature, using various catalysts including fluorinated chromia. Equation (4) is a fluorinated chromia-catalyzed reaction receiving... 

Many different reaction schemes, as described in Figure 7.5, are considered and both homogeneous and heterogeneous catalysts are featured. Fluorinated chromia appears as the catalyst for the catalytic fluorination step that leads to the intermediate, CF3C1C=CH2, HFO-1233xf it is apparent that catalyst deactivation is a problem and for this reason addition of a basic molecule such as di-isopropylamine to the vapor feed is recommended. 

The derivation of a mechanism for a chemical reaction is by its very nature an uncertain process, being dependent critically on the nature and extent of the experimental evidence. Mechanisms that have at their heart a surface process or processes are even more uncertain and when the constraints imposed by the manipulation of HF are also taken into account, it is not surprising that there have been relatively few mechanistic studies made of heterogeneous catalytic fluorination. However a catalytic process cannot be said to be understood fully without a mechanism based on the experimental evidence available and such studies are helpful in the design of the next generation of catalysts. In most cases the work described below involves chromia or y-alumina based catalysts that have been pretreated according to the methods described above. Studies involving C2 and Q compounds are described in turn. 

The main synthetic route to CFC, HCFC and Halons is the Swarts fluorination. Technically this is often achieved by reaction of a chlorinated or brominated precursor with anhydrous hydrofluoric acid in the presence of a solid Lewis acid catalyst, for example chromia. Other important reactions are Lewis acid-catalyzed halogen isomerization and hydrogenolysis of chlorine or bromine. 

Because of the corrosive nature of HF, there are few reports of fluorination reactions in the liquid phase using solid catalysts. The use of zeolites in fluorination was reviewed in our earlier publication [1]. An unusual, room-temperature, solid-catalyzed fluorination of CH3CCI3 and related species by anhydrous HF has been reported by Thomson et al. [92-94]. He found that fluorination at ambient temperatures is possible in the presence of catalysts derived from fluorinated y-alu-mina, chromia, Fc304, and C03O4 and conditioned before reaction by treatment... 

Moore and Massey [61] detailed a reaction in which 1,1,2,2-tetrafluoroethane was isomerized to 1,1,1,2-tetrafluoroethane over chromia catalyst at 400°C, indicating that fluorine shifts may be catalyzed. The same authors also indicated that AIF3 and NaF may also be used as catalyst. Similar work for TFE has not been reported, though it stands to reason that such a shift would be beneficial to the formation of HFP (See Scheme 5.7). 

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